An organic EL element can manufacture a display element and lighting which are driven by low power, are thin and light, and have excellent flexibility, and has been therefore actively studied as a next generation light emitting display element.
An organic EL element has a structure having a pair of electrodes composed of a positive electrode and a negative electrode, and a single layer or a plurality of layers which are disposed between the pair of electrodes and contain an organic compound. Examples of a layer containing an organic compound include a light emitting layer and a charge transport/injection layer for transporting or injecting a charge such as a hole or an electron. As a method for forming these organic layers, a vacuum deposition method or a wet film formation method is used.
The vacuum deposition method is advantageous in that a high-quality film can be formed uniformly on a substrate, a luminescent element which can be easily laminated and has excellent characteristics can be easily obtained, an extremely small amount of impurities derived from a manufacturing process are mixed, and the like. Many organic EL elements which are practically used now are obtained by the vacuum deposition method using a low molecular weight material. Meanwhile, a vacuum deposition apparatus used in the vacuum deposition method has such problems that the apparatus is generally expensive, continuous manufacturing is difficult, and manufacturing cost is high when all the steps are performed in vacuum.
On the other hand, the wet film formation method does not require a vacuum process, does not require an expensive vacuum deposition apparatus, and therefore makes it possible to form a layer at relatively low cost. In addition, the wet film formation method is advantageous in that an area can be large, continuous manufacturing is possible, a plurality of materials having various functions can be contained in one layer (coating liquid), and the like. Meanwhile, in the wet film formation method, lamination is difficult, and it is difficult to obtain a high-quality and uniform coating film which does not contain impurities derived from a manufacturing process (for example, a solvent).
For the wet film formation method, ink compositions utilizing phosphorescent light or thermally activated delayed fluorescence have been developed (Non Patent Literatures 1 and 2), but in general, a host compound and a dopant compound constituting these ink compositions have a low solubility, film formability is not good, and optimization of these compositions has not been progressed. Therefore, at present, a characteristic comparable to an organic EL element using the vacuum deposition method has not been achieved yet.